1. Field of the Invention
The present invention relates to a radio wave propagation characteristic estimation apparatus and a computer program.
Priority is claimed on Japanese Patent Application No. 2008-242516, filed Sep. 22, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
In a wireless communication system such as a mobile phone network, there are conventionally known a variety of techniques where in order to estimate a communicable region (a service area) of a given base station, a ray tracing method is used to calculate radio wave propagation characteristic estimation data (a trace of a radio wave, a propagation loss, a received signal intensity, and the like) in which a characteristic related to propagation of a radio wave launched from one base station is estimated. The ray tracing method is disclosed in, for example, Non-Patent Document 1, “Hosoya, Yoshio (Ed.), Radiowave Propagation Handbook, Part 2, Chapter 15, 15.5, 1999, Realize Science & Engineering Center Co., Ltd.”.
The ray tracing method includes an imaging method and a ray launching method. The imaging method calculates a trace of reflections and diffractions of a radio wave on the mirror image principle. According to the imaging method, it is possible to correctly find a trace of a radio wave. However, with an increase in number of the reflection surfaces and the diffraction points, an amount of calculation increases exponentially. In the ray launching method, launch directions of the radio waves at a transmission point are discretely provided. Radio waves are launched in the launch directions and the traces of the radio waves are sequentially followed. According to the ray launching method, it is possible to suppress an amount of calculation more than the imaging method. However, it is difficult to take diffraction waves into consideration.
In the technique described in Patent Document 1, “Japanese Unexamined Patent Application, First Publication No. 2005-318308”, the ray launching method is used to calculate a trace of a radio wave over a plurality of floors within a building. In the technique described in Non-Patent Document 2, “KOZO KEIKAKU ENGINEERING Inc., “RapLab”, [searched on Aug. 21, 2008], the Internet <URL: http://www4.kke.co.jp/raplab/>”, the imaging method is used to calculate a trace of a radio wave. Non-Patent Document 3, “Information and Mathematical Science Laboratory Inc., “Comparison between Launching Method and Imaging method”, [searched on Aug. 21, 2008], the Internet <URL: http://www.imslab.co.jp/Product/eem/rtm/raytrace.htm>” describes a comparison between the ray launching method and the imaging method, in which is disclosed a system of calculating a propagation route with a small number of reflections by the imaging method while calculating a propagation route with a large number of reflections by the ray launching method.
As a conventional technique for estimating a received signal intensity of a radio wave from every base stations on outdoor main roads, there is known a technique disclosed in, for example, Non-Patent Document 4, “ERICSSON, “Planet EV”, [searched on Aug. 21, 2008], the Internet, <URL: http://www.ericsson.com/solutions/tems/network_plan/planetev.shtml>”.
The technique of Non-Patent Document 4 relates to an area design tool for mobile phones, in which the coverage and the influence of interference by a plurality of base stations over a wide area is estimated. Its main object is to provide a two-dimensional wireless area design. To be more specific, representative values of a building environment in the area such as an average building height and an average road width are extracted. Then, the statistical method (an estimation formula) described in Chapter 15 of Non-Patent Document 1 is used to estimate a signal intensity from every base station. The technique of Non-Patent Document 4 uses map data and the like in which a map database including building data is processed and an average height of buildings for, for example, every 25 square meters is extracted.
However, in the aforementioned conventional techniques, the ray launching method and the imaging method are not properly combined.
The conventional techniques described in Patent Document 1 and Non-Patent Document 2 uses only one of the ray launching method and the imaging method, and does not combine the ray launching method with the imaging method.
Non-Patent Document 3 discloses a system of applying the imaging method to a propagation route with a small number of reflections while applying the ray launching method to a propagation route with a large number of reflections. However, it is necessary to take the trouble to select the use of either the ray launching method or the imaging method for every propagation route based on the number of reflections. This is inefficient. In general, traces of radio waves can be found more correctly by the imaging method than by the ray launching method. However, if selection of use is made between the ray launching method and the imaging method according to the number of reflections, it follows that simply because the number of reflections is large (the number of reflections is a specified number or greater), the ray launching method is applied to a propagation route to which the imaging method is preferably applied. This is very inefficient.
For example, as shown in FIG. 9, among the propagation routes of the radio waves launched from a transmission point (a base station) at a high elevation position to roofs of the buildings in the direction of the ground, there is a propagation route that repeats reflections and diffractions from roof to roof of the buildings. In this case, it is desirable that the trace of the radio wave repeating reflections and diffractions from roof to roof of the buildings be correctly found by the imaging method in consideration of diffraction waves. However, if selection of use is made between the ray launching method and the imaging method according to the number of reflections, there is a possibility that the ray launching method is adopted, thus failing in correctly taking the diffraction waves into consideration.
Furthermore, there is known a conventional technique for estimating a radio wave characteristic from an outdoor base station (a transmission point) to an outdoor mobile station (a reception point) (for example, Non-Patent Document 4). However, a problem of estimating a radio wave propagation characteristic from an outdoor base station (a transmission point) to an indoor mobile station (a reception point) persists.
The present invention has been achieved in view of such circumstances, and has an object to provide a radio wave propagation characteristic estimation apparatus and a computer program that properly combine a ray launching method and an imaging method to thereby make it possible to efficiently generate radio wave propagation characteristic estimation data and also to estimate a radio wave propagation characteristic from an outdoor transmission point to an indoor reception point.